1. Field of the Invention
The present invention relates to a sensitive coordinate input device which is capable of changing a coordinate signal based on variation of electrostatic capacitance, and particularly to a sensitive coordinate input device adopting a system in which an operator's finger is regarded as one electrode, and an electrostatic capacitance is varied depending on a change in the distance between the finger and another electrode which is separated therefrom by a dielectric.
2. Description of the Prior Art
Conventional coordinate input devices include a mouse, a track ball, a controller, a cursor key, and the like, and also a sensitive coordinate input device.
Prior art examples of such a sensitive coordinate input device is shown for explanation in FIGS. 3, 4, and 5.
The sensitive coordinate input device shown in FIGS. 3 and 4 comprises four electrodes 1, 2, 3, and 4 which are closely arranged at regular angular positions. A dielectric film 5 is disposed so as to cover these electrodes 1 to 4. The sensitive coordinate input device converts variation of electrostatic capacitance which is caused by pressing the dielectric film 5 with an operator's finger F or by moving the finger F on the dielectric film 5, into a coordinate signal by a conversion circuit which is not shown.
The sensitive coordinate input device shown in FIG. 5 is described in Japanese National Patent Publication No. 4-507316 (WO 91/03039). In this sensitive coordinate input device, a plurality of columns of first electrodes 6 convert variation of electrostatic capacitance which is caused by making the operator's finger F contact with the second electrodes 7, into a coordinate input signal by a conversion circuit which is not shown.
In the sensitive coordinate input device described with reference to FIGS. 3 and 4, the electrostatic capacitance is varied in accordance with the increase or decrease of the contact area in which the finger F is in contact with the dielectric films. Therefore, in the case where the electrodes 1 and 2 are assigned to indicate the X axis on the screen of a CRT display apparatus, and the electrodes 3 and 4 are assigned to indicate the Y axis thereon, in order that the movement of the cursor on the screen of the CRT display apparatus always follows the movement of the finger F on the dielectric film 5, it is necessary to closely dispose the four electrodes 1 to 4 as shown in FIG. 3.
The sensitive coordinate input device described with reference to FIG. 5 is considered to adopt a system in which the electric field is disturbed by depressing the second electrodes 7 with the finger F, thereby varying the electrostatic capacitance C. Accordingly, in the case where two of the four overlap portions formed by the columns of first electrodes 6 and the rows of second electrodes 7 are assigned to indicate the X axis on the screen of the CRT display apparatus, and the remaining two overlap portions are assigned to indicate the Y axis thereon, in order that the movement of the cursor on the CRT display apparatus always follows the movement of the finger F on the second electrodes 7, it is necessary to closely dispose the plural columns of first electrodes 6 and the plural rows of second electrodes 7, in the same way as in the prior art example shown in FIGS. 3 and 4.
However, in a sensitive coordinate input device in which electrodes must be closely disposed as in the above-described two examples, if attempts to improve operability by increasing an area of a sensitive area, i.e., an electrode formation area, the will have to be modified in the following manner. In the example shown in FIGS. 3 and 4, the sizes of the electrodes 1 to 4 must be increased, and, in the example shown in FIG. 5, the number of columns of first electrodes 6 and the number of rows of second electrodes 7 must be increased. In the example shown in FIGS. 3 and 4, the increase of the size of each of the electrodes 1 to 4 inevitably results in an increase in cost for forming electrodes on a substrate. In the example shown in FIG. 5, an increase in the number of columns of first electrodes 6 and rows of second electrodes 7 inevitably results in a complicated construction. For these reasons, the sensitive area is conventionally confined to a small rectangular area having a side of about 20 mm, or to a small circular area having a diameter of about 20 mm. Such a small sensitive area, however, hinders improvement in operability.
The contact area with the finger F is varied because of various magnitudes of forces exerted when the finger F touches the dielectric film 5 or the second electrodes 7, and accordingly the electrostatic capacitance is varied. Thus, there exists a problem in that it is difficult to determine the position of the cursor on the CRT display apparatus.